1. Field of the Invention
This invention relates to the optimization of concentrate flow in a filter system and more particularly to a system using actual sensed operating curve data to optimize plasma flow in a plasmapheresis system.
2. Discussion of the Prior Art
Conventional filtering systems involve a tripartite fluid flow relative to a porous membrane. Feed fluid which is to be filtered is presented to a first side of the membrane and flows across the first side of the membrane along a longitudinal axis of the filter system. Filtrate fluid passes through the membrane and is withdrawn from the second side of the membrane, opposite the first side. Components of the feed fluid which pass along the membrane without passing there-through are drawn off as a retentate or concentrate fluid.
In a typical filtering application such as a plasmapheresis system wherein plasma is separated from whole blood to form a packed cell concentrate, it is desirable to maximize the percentage or absolute flow rate of the filtrate. For moderate filtrate flow rates and fixed membrane area, the filtrate flow rate is approximately proportional to transmembrane pressure (TMP).
However, as the filtrate flow rate increases a reversible blocking effect causes transmembrane pressure to increase more rapidly relative to filtrate flow. The blocking effect is reversible in the sense that if the filtrate flow is decreased after blocking has occurred there is no plugging of the filter pores and the original substantially linear TMP-filtrate flow relationship is reestablished.
However, if the filtrate flow (and TMP) become sufficiently high, red cells, platelets or other particulate matter lodge permanently in the membrane pores and begin to irreversibly plug the filter. The plugging decreases the effective area of the membrane and if continued over a period of time will cause the filtrate flow rate to decrease while the TMP remains constant or even increases, or cause an increase in TMP if the filtrate flow rate is maintained. If the filtrate flow rate is decreased the filter membrane remains partially plugged and the effective area of the membrane is permanently decreased wherein the original TMP-filtrate flow relationship is changed undesirably.
One filter flow control system is partially described in Lysaght, M. J.; Schmidt, B.; Samtleen, W.; and Gurland, H. J. "Transport Considerations in Flat Sheet Microporous Membrane Plasmapheresis," Plasma Therapy Transfusion Technology, Vol. 4, No. 4 (1983) pp. 373-85. The described system uses a pumping system with pumps driving the feed fluid (blood) and filtrate (plasma). The control system senses pump flow rates in the feed fluid and filtrate paths and senses pressure in all three filter fluid paths. A clamp also controls the flow of feed fluid over the membrane surface. The sensed information is used by an undisclosed control algorithm to control the filtrate flow and clamp to keep TMP constant and to maintain a desired inlet to outlet pressure differential.
Some systems use a capillary separator in place of a flat membrane device. The capillary separator uses hollow fibers with thin, porous walls. The walls of the fibers are essentially porous membranes and function in a manner similar to a flat membrane. A system using a capillary separator is described in Buchholz, D. H.; Porten, J.; Anderson, M.; Helphigstine, C.; Lin, A.; Smith, J.; Path, M.; McCullough, J.; and Snyder, E.; "Plasma Separation Using the Fenwal COS-10 Capillary Plasma Separator," Plasmapheresis, edited by Y. Nose, P. S. Malcesky, J. W. Smith and R. S. Krakauer, Raven Press, New York (1983).